This application claims the priority of Korean Patent Application No. 2002-83202 filed Dec. 24, 2002 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an apparatus and method of calibrating the marking position of a chip scale marker, and more particularly, to an apparatus and method of calibrating the marking position of a chip scale marker which marks a character on a wafer chip using a laser beam.
2. Description of the Related Art
Several thousands or tens of thousands of chips are formed on a wafer used in a semiconductor process. To classify the chips according to production lots, characters and/or numbers are marked on a surface of each chip. A chip scale marker using a laser beam is used as an equipment for marking.
FIG. 1 is a view illustrating a constitution of a typical chip scale marker. FIG. 2 is a view illustrating a constitution of the laser system of FIG. 1.
Referring to FIGS. 1 and 2, a wafer w is placed on a wafer holder 20 and a laser system 30 is arranged under the wafer holder 20. A laser beam oscillated by a laser oscillator 31 of the laser system 30 is magnified by a beam expander 32 and input to a Galvano scanner 33. The laser beam input to the Galvano scanner 33 is irradiated onto chips on the wafer w through an f-xcex8 lens 34 so that characters are recorded on a surface of the chips. The above laser system is disclosed in Japanese Patent Publication No. H9-248692.
A camera 40 monitoring the wafer w being supported by the wafer holder 20 is arranged above the wafer holder 20. The camera 40 is moved by being connected to an X-Y stage 50.
FIG. 3 is a view illustrating that a marking shape is distorted by the Galvano scanner. The Galvano scanner 33 includes an x mirror 33a and a y mirror 33b. The x mirror 33a controls movement of a laser beam in a direction x by an x drive (not shown) rotating a shaft 33c at one end of the x mirror 33a. The y mirror 33b controls movement of a laser beam output from the x mirror 33a in a direction y by a y drive (not shown) rotating a shaft 33d at one end of the y mirror 33b. Thus, an optical path in the direction x is longer than that in the direction y. Accordingly, when a signal to mark a rectangular shape as shown in FIG. 4A is transmitted, a distortion like a pin cushion as shown in FIG. 4B is generated. Also, a positional error is generated due to a tiny difference in position between center lines of rotation shafts 33c and 33d of the x and y mirrors 33a and 33b and surfaces of the x and y mirrors 33a and 33b. 
In the meantime, as the beam having passed through the Galvano scanner 33 passes through the f-xcex8 lens 34, the beam is curved so that a barrel distortion is generated. To improve the distortion phenomenon of the marking, marking calibration should be periodically performed to control the rotation of the x and y mirrors 33a and 33b of the Galvano scanner 33.
FIG. 5 is a view illustrating a conventional method of measuring a marking error. According to the conventional method, a laser beam is irradiated onto a plate 70 having a shape and size corresponding to a wafer and a plurality of small holes 70a having a diameter of 0.3 mm formed in the plate 70 at a predetermined interval, and the position of the laser beam passing through each of the holes 70a is observed by the camera 40 and compared with a target position of the laser beam. Next, a degree of an error in the irradiation position of a laser beam is recognized and a path along which the laser beam is irradiated is corrected.
However, in the conventional method, since the laser beam passing through the holes 70a is observed through a glass portion 42 in front of the camera 40, a laser beam inclined with respect to the holes 70a, as indicated by a dotted line of FIG. 5, is refracted by the glass portion 42 of the camera 40. Thus, an accurate position on the plate 70 where the laser beam is irradiated is difficult to recognize and it takes some time for the camera 40 to move above the holes 70a to be measured. Also, the plate 70 may be damaged by power of the laser beam.
To solve the above and/or other problems, the present invention provides an apparatus and method of calibrating the marking position of chip scale marker by radiating a laser beam onto a screen corresponding to a wafer by reducing power density of the laser beam in use by using a pinhole apparatus and measuring the irradiated laser beam.
According to an aspect of the present invention, a chip scale marker including a laser system, a wafer holder supporting a wafer to be processed, and a camera moving above the wafer holder by being connected to an X-Y stage and monitoring the wafer supported on a center hole of the wafer holder, the chip scale marker comprising a unit detachably arranged on a laser beam path from the laser system and reducing power density of a laser beam; and a screen arranged on a center hole of the wafer holder and indicating a position where a laser beam from the laser system is irradiated.
The laser beam power density reducing unit is a pinhole apparatus having a pinhole having a predetermined diameter. The chip scale marker further comprises an ND filter reducing the quantity of the laser beam at a predetermined rate.
The pinhole apparatus is manufactured of invar or diamond. The pinhole apparatus has a convex surface in a direction in which the laser beam is input. In the pinhole apparatus, the diameter of the pinhole increases along the laser beam path.
The screen comprises a lower layer absorbing the irradiated laser beam and an upper layer transmitting the light from the lower layer upward in a vertical direction.
The screen comprises a lower layer made of glass or acryl whose surfaces are roughly processed to disperse light at a point where the laser beam is irradiated and an optical attenuator arranged above the lower layer to provide a single point upward by filtering the dispersed light.
The screen is a semi-transmissive glass or paper.
The wafer holder further comprises a plurality of holes formed on a concentric circle separated a predetermined distance from the center hole of the wafer holder and a semi-transmissive film provided on the holes.
A chip scale marker including a laser system, a wafer holder supporting a wafer to be processed, and a camera moving above the wafer holder by being connected to an X-Y stage and monitoring the wafer supported on the wafer holder, the chip scale marker comprising a unit detachably arranged on a laser beam path from the laser system and reducing power density of a laser beam; a camera screen arranged in front of the camera; and a unit installing and removing the camera screen at and from a front side of the camera.
The camera screen installing and removing unit is a unit rotating the camera screen.
The camera screen is a paper roller supported by two support shafts so that, as a first support shaft rotates, paper wound around a second support shaft is released to be wound around the first support shaft.
The camera screen installing and removing unit reciprocates the camera screen in a direction along the support shafts.
According to another aspect of the present invention, a method of calibrating a marking position of a chip scale marker including a laser system, a wafer holder supporting a wafer to be processed, a camera moving above the wafer holder by being connected to an X-Y stage and monitoring the wafer supported on the wafer holder, a unit detachably arranged on a laser beam path from the laser system and reducing power density of a laser beam, and a screen arranged on a center hole of the wafer holder and indicating a position where a laser beam from the laser system is irradiated, the method comprising the steps of radiating a laser beam using the laser system to a target position on the screen; measuring a position of the laser beam irradiated to the screen; and calibrating the laser system by comparing the measured position of the laser beam and a target position, wherein the screen is made of paper, and the position of the laser beam is a position where the screen is changed black by the laser beam irradiated by the laser system whose power density is reduced by the laser beam power density reducing unit.